1. Field of the Invention
The present invention relates to a process for separating organic liquid mixtures, such as alcohols from mixtures with water, using sulfonated ion exchange polyalkylene (polyalkene) membranes.
2. Description of Prior Art
Azeotropic mixtures or close boiling mixtures are separated at present on a large scale by multistage distillation (rectification) or, sometimes, by a combined process such as extractive distillation. These separation processes are characterized by: high energy demands, relatively large capital plant investments, a variety of maintenance problems, and severe environmental problems that contribute to the energy demands. For example, high temperature multistage distillation increases thermal loads on the cooling system of a chemical plant and generally contributes to water and air pollution. Government regulations, which are designed to mitigate these problems, often constitute an additional energy burden on the separation process.
The need to use energy efficiently, combined with the sharp rise in the cost of petroleum products, have focused attention on by products of the chemical and petroleum industries. Most of these by products consist of mixtures of liquid organic compounds, whose separation is often complicated and costly. Therefore, some of the by products have been considered, until recently, as expendable and disposable, or were burned to provide very expensive energy. The development of a low-cost technique of separating such mixtures would clearly be of great benefit.
Separation processes which involve the use of porous and/or semi-permeable membranes for separating compounds from each other have been used for solving many problems. For example, membrane separation processes have been applied in biotechnology. Conventional separation techniques such as distillation, adsorption, liquid-liquid extraction and crystallization are often insufficient and uneconomical. Application of membrane technology can save in process costs because energy consumption is low, raw materials and nutrients can be recovered and reused, fermentation processes can be carried out continuously, and disposal problems can be reduced or eliminated. See, Lee et al., ("Membrane Separations in Alcohol Production", 369 Ann N.Y Acad Sci. 367 (1981), who propose to achieve cost savings by using membrane processes, such as ultrafiltration, reverse osmosis and electrodialysis, in combination with distillation.
Membrane permeation by the pervaporation process involves selective sorption of a liquid mixture into a membrane, diffusion through the membrane, and desorption into a vapor phase on the permeate side of the membrane. Because of the interesting potential applications of pervaporation techniques, for example, to the separation of organic liquid mixtures, attempts have been made to discover commercially acceptable membranes.
Binning et al., 37 Pet. Refiner. 214 (1958); and Binning et al., 3(1) E.C. Am. Chem. Soc. Div. Pet. Chem. Prepr. 131 (1958) describe separation of organic liquid mixtures by synthetic membranes, and in particular, the economics of drying 2-propanol via a selective synthetic membrane, claiming that the membrane separation process was more feasible than a conventional azeotropic distillation with hexane. Elaborate analyses of such processes are described in Choo, 6 Pet. Chem. 73 (1962). Numerous studies dealing with separating organic liquids via pervaporation processes have been reported. Nevertheless, this technology has not kept pace with other membrane separation processes such as reverse osmosis, electrodialysis and hemodialysis, which have been commercialized. The reasons, among others, have been the poor separations obtained from commercial films, e.g., polyethylene (See Huang et al. 12 J. Appl. Polym. Sci. 2615 (1968)), that served as membrane components, and the lack of proven feasibility of the conventional pervaporation process.
Mulder et al. "Ethanol Water Separation by Pervaporation", 16 J. Membr. Sci. 269-284 (1983) summarize selectivities and permeation rates when dense homogeneous membrane materials were used to separate ethanol-water mixtures reported in the literature. The membrane materials include cellulose acetate, cellulose, polytetrafluoroethylene (PTFE)-polyvinylpropylene, cellophane, PTFE-polysulfone, polyethylene, polyethylenetetrafluoride, and polyvinylalcohol. The reported selectivities and permeation rates for these materials were very low with selectivities from less than 0.0006 to 11 and permeation rates from less than 0.01 to 9.46 (cm/hr). Mulder et al. also report selectivities and permeation rates of other homogeneous membrane materials they tested at thickness from 10 to 30 micrometers, including cellulose acetate, cellulose triacetate, cellulose tripropionate, cellulose acetate butyrate, polyacrylonitrile (PAN), polyvinylidenefluoride, polysulfone (PS) and polydimethylsiloxane. The reported selectivities and permeation rates for these other materials except for PAN and PS also was very low with selectivities from 0.3 to 4.1 and permeation rates from 0.017 to 0.113 (cm/hr). The selectivities reported for PAN (70) and PS (332) appear high, but combined with the low permeation rates (flux through the membrane) reported for PAN (0.0015 cm/hr) and PS (0.0004 cm/hr) render such membrane materials of little or no practical utility.
Pervaporation membranes for selective separation of water from aqueous starting mixtures containing organic and inorganic dissolved constituents are mentioned in Chem. Abstr., Vol. 76, 1972, No. 117314r, page 310 (nitrogen-free membrane containing at least one homo-, or copolymer of at least one polyester, polyketone, polysulfone and may contain maleic anhydride, maleic acid, vinylsulfonate, vinylbenzoic acid); and in Chem. Abstr., Vol. 72, 1972, No. 131247c, page 296 (membrane made of mixed polymers of acrylonitrile and acrylic acid).
The use of three types of dense isotropic membrane materials: (1) alloys of polyphosphonyl esters and cellulose acetate; (2) a highly cross-linked quarternary derivative of polyphenylene oxide; and (3) an ion exchange membrane consisting of sulfonic acid groups attached to a fluorohydrocarbon matrix (copolymers of polysulfonyl fluoride vinyl ether and polytetrafluoroethylene (PTFE)) (available as Nafion 125.TM. from E. I. DuPont DeNemours & Co., Wilmington, Del.), as pervaporation membranes is reported in Cabasso, 22 Ind. Eng. Chem. Prod. Res. Dev. 313 (1983). The membranes were used for the separation of four liquid mixtures: (1) methanol/benzene 26.9/73.1 (percent composition in azeotrope); (2) benzene/cyclohexane, 55/45 (percent composition in azeotrope); (3) cyclohexane/cyclohexane; and (4) styrene/ethylbenzene, composed of a copolymer of polysulfonylfluoride vinyl ether and polytetrafluoroethylene blocks.
Cabasso reports, for example, for the separation of methanol-hexane mixtures, separation factors higher than 50 accompanied by high permeation were attained by employing an ion-exchange Nafion 811.TM. membrane
U.S. Pat. No. 2,953,502 describes the separation of benzene from an azeotropic mixture of benzene and methanol by means of a non porous polyethylene membrane. U.S. Pat. No. 4,073,754 to Cabasso et al. describes the use of pervaporation membranes for separation of aromatic hydrocarbons such as benzene, cyclohexane, ethanol, from other organic solvents, e.g., aliphatic hydrocarbons such as cyclohexane, decalin, heptane. The membrane materials employed consisted of a polymer alloy of poly (vinylidenechloride-benzyldiethyl phosphone) copolymer and acetyl cellulose; a phosphonylated poly(phenylene)oxide derivative and acetyl cellulose; and poly(2-methyl-6-methylenedimethylphosphonate-3-bromo-1, 4-phenylene) oxide.